Piezoelectric materials are important for many technological fields of application. Here materials are involved which generate electric charges in response to the effects of a mechanical load or change their shape in response to the effects of an electric field. The first aspect is known by the term “direct piezoelectric effect” whilst the latter case is referred to as “indirect piezoelectric effect”.
The piezo ceramics are a special group of piezoelectric materials. For the production of piezo ceramics, the piezoelectric effect must be caused to occur selectively by polarising the ceramic material after vitrification in a strong electric field. In this operation, defined areas of the ceramic material, the domains, are oriented along the lines of the electric field in order to polarise the ceramic material in this direction. After the electric field has been switched off, one part of this polarisation is retained as remanent polarisation in the ceramic material. Hence, this polarising operation turns an isotropic material into an anisotropic material having piezoelectric characteristics. When an electric field is applied to the ceramic material, for example in the polarising direction, this material extends in the direction of the field (d33 effect) whilst it contracts in a direction orthogonal on the field direction (d33 effect). The type of deformation is hence dependent on the direction of the applied field relative to the polarising direction of the ceramic material. The 3-1 effect corresponds approximately to only ⅓ of the 3-3 effect, which is the deformation in the field direction. When piezoelectric ceramic materials are employed as actuators in technology, it is therefore envisaged, as a rule, to utilise the 3-3 effect.
Piezoelectric ceramic materials can also be used, of course, as sensors when the direct piezoelectric effect is utilised. In such a case, a deformation of the ceramic material generates an electric charge whose magnitude depends, inter alia, also on how the deformation takes place relative to the direction of the remanent polarisation of the ceramic material.
Lead zirconate titanate (PZT) is the piezoelectric material that is most frequently used in industry at present. This ceramic material has been used in technology for a long time in many different ways as resonator, actuator or sensor material.
Finely scaled components such as films, tiny rods or fibres are expedient for many fields of application. They entail, however, the problem of low mechanical strength of the piezoelectric ceramic materials to be made available in this form. This applies to both their handling in the manufacture of complex structures made of these ceramic materials and the limited possibility of power transmission by means of the piezoelectric effect into a component.
To overcome these problems, therefore composite materials are frequently used which consist of an organic polymer matrix with a PZT ceramic material embedded therein. On the one hand, the polymer matrix serves the purpose of endowing the thin-walled ceramic elements with a sufficient mechanical stability and, on the other hand, it has the function of transferring the deformations created by the piezo ceramic material into the structure, for instance, when vibrations are attenuated.
For example, an actuator with a composite design has become known from the U.S. Pat. No. 5,687,462, wherein vitrified piezo ceramic films are integrated into an organic matrix. The robust components so created may be used both for sensor and actuator functions in oscillating systems. There, the matrix endows the brittle PZT ceramic material with mechanical strength whilst it serves also to transfer power into a major structure. In these elements, the electric field is usually applied to the PZT film in the direction of the thickness so as to utilise the 3-1 effect. It is also possible to apply inter-digital electrodes on the PZT film for utilisation of the 3-3 effect for actuator applications. In these cases, however, high field gradients occur at the electrode corners in ceramic films, which may result in great strain in the ceramic material and in fracture of the monolithic piezo ceramic material.
More recently, therefore composite elements have also been used in addition to the PZT films, wherein active piezoelectric fibres are embedded in a polymer matrix. These systems can be operated both with utilisation of the 3-1 effect and—by means of inter-digital electrodes—with utilisation of the 3-3 effect.
Compared against composite structures, these composite fibre structures with a monolithic PZT film present several advantages. For example, firstly, an extension in the longitudinal direction of the fibres is mainly induced due to the anisotropy of the fibres, and secondly the fibres can be integrated throughout the structure with a wide extension. An adaptation to a three-dimensionally shaped structure is easier to realise with the fibres than with a monolithic PZT film.
A major problem in the manufacture of composite structures consisting of piezoelectric fibres resides, however, in the high tendency of these fibres to rupture, which requires specific devices for handling them. For example, the U.S. Pat. No. 5,869,189 describes a method of manufacturing composite fibre structures, where this problematic aspect becomes clear. In the method proposed there, several elongate piezoelectric fibres are placed into a mould in parallel and at a defined spacing from each other. To ensure the correct spacing and the parallel orientation of the fibres a comb-like structure may be provided on the bottom of this mould. Subsequently, a liquid polymer material is charged into the mould and allowed to cure together with the fibres. The polymer matrix so created presents substantially the thickness of the single layer of the fibres. Then, the surface of this polymer matrix is slightly grounded. This serves firstly the purpose of roughening the surface and secondly to expose some regions of the fibres. Finally, metallic silver is deposited as electrode layer on the surface of the matrix. Due to the previous roughening, the adherence of this electrode layer is improved and because of the exposure of isolated regions of the fibres, a direct electrical contact with the fibres can be established.
A substantial problem in the manufacture of this composite fibre structure consists, however, in the insertion of the long piezoelectric fibres in a parallel arrangement in the mould envisaged to this end. This involves the high risk of fracture of many fibres. Moreover, the required parallel orientation is linked up with a substantial expenditure in manufacture. The provision of very straight fibres, which is required to this end, is problematic, too, because fibres are often prone to bend in manufacture and can therefore no longer be oriented in parallel. This applies particularly to the case of very thin fibres having a diameter of <50 μm.
Another method of manufacturing composite fibre structures is known from the German Patent DE 198 29 216 A. There, the fibres are directly contacted by the electrodes via their periphery whilst they are enclosed at least partly. This is achieved by fixing the fibres in a conductive adhesive material and by potting them subsequently. This involves the problem, however, that the adjacent fibres must not be too close to each other or else the adhesive substance is drawn into the space between the fibres, due to the capillary effects, before it is cured. As a result, the electric disruptive strength is strongly reduced while the probability of disruptive breakdown by the time of polarisation of the fibres or in application as actuators is very high. Moreover, in this method, too, the difficulties in handling the extremely fragile piezoelectric ceramic fibres continue to exist.